These jackets may be of considerable length, 200 meters being now fairly commonplace. To achieve stability at such a length they are usually tapered from a wide base to a narrower apex. The base may be as much as 100 meters across.
Because of their enormous size and weight these jackets are usually built on their sides on a slipway leading to water, so that a launching barge can be brought up close to the place of building. Once built, they are slid lengthwise onto the barge and taken to where they are to be used. Except where stated otherwise, references in this specification to "height", "vertical", "length" etc all relate to the situation at the construction site, with the jacket lying on its side.
The slipways are much closer together than the width of the base of the jacket--otherwise they could not continue onto any reasonable barge. So a specific internal structure is provided in the jacket which primarily acts to support and sustain the jacket while it is lying on its side and supported only on the slipways. The portions of the jacket laterally to each side of this special structure hang right out beyond the support points on the slipway.
The conventional method of construction and assembly of all these parts is to prefabricate them while they are lying flat on the ground and then erect them into the vertical condition by means of cranes. This process is called rolling up. They are then held vertical while the relevant braces are fitted in order to hold them together in the final assembly. First, the two trusses (that is to say basically planar structures extending lengthwise along the jacket) constituting the special internal structure are built lying flat and are then rolled up together to form the internal rectangular structure. The trusses are sometimes known in this art as "bents".
Thereafter, the outer trusses which are to define the side walls of the jacket are similarly built flat and rolled up and secured to the internal structure by braces. Then, temporary supports on the ground which have held the outer trusses are removed and the completed structure is ready to be moved to the barge.
The difficulty is to lift during the rolling up structures of this very considerable size and (when vertical) height.
Typically, the problem has been that the jacket designer has been concerned primarily with the finished product, from the point of view of length, stiffness requirements and so forth and has been concerned secondarily with the problems of the constructor (fabricator) of such jackets. The limits have been reached of the conventional methods of constructing these objects and a totally different approach to their design is needed, one which will take account of the problems of assembly and construction just as much as the problems of eventually providing a jacket which when on site will work to a given specification.